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ZTE's Distributed Soft BTS Solution

2008-03-24
Mou Yongjian

Requirements for 3G BTS EquipmentWith the increase of voice traffic and wide applications of high-speed packet data services, larger capacity for voice services and broader bandwidth for data services are required. To meet these requirements, most traditional mobile operators have to construct 3G networks based on their current 2G networks. However, the Base Transceiver Station (BTS) equipment used in the existing 2G networks is generally unable to be directly upgraded to support 3G services. As a result, new BTS equipment is needed in constructing 3G networks.

One critical problem most operators face is selecting a proper site and finding enough installation space for the new BTS equipment. Most operators have difficulties in site selection, and their equipment room spaces are often insufficient. Therefore, among others, an important requirement for 3G BTS equipment is that it should be designed to save the installation space and be flexibly applied in different installation scenarios.

Another problem in 3G network construction is standard selection. At present, there are three mainstream 3G standards: CDMA, WCDMA and TD-SCDMA. Each standard has several versions used for different evolutionary steps. For example, the CDMA2000 1xEV-DO involves Rel.0, Rev.A and Rev.B. Most operators have already decided which 3G standard is to be used, but as to the evolution from 3G to 4G (UMB, LTE and WiMAX), or to the transitional releases (e.g., EVDO Rev.B), they are still adopting a wait-and-see attitude. Before investing in 3G networks, they have to consider how to evolve their networks, hoping that the evolution route they choose can facilitate a graceful and lowest-cost upgrade from 3G to 4G in the future. Hence, the BTS equipment for 3G networks should be upgradeable to support the 4G standard the operators choose, thus, better protecting their investment in 3G networks.

ZTE's Distributed Soft BTS SolutionTo solve the above problems, ZTE first proposed the distributed BTS solution in the telecom industry. Unlike the traditional centralized BTS, the distributed BTS physically separates its baseband unit and radio frequency (RF) unit and connects them via the standard baseband/RF interface (e.g., Common Public Radio Interface/Open Base Station Architecture Initiative, or CPRI/OBSAI). As shown in Figure 1, the baseband unit and the RF unit of the traditional BTS are separated into two independent function modules in the distributed BTS: Base Band Unit (BBU) and Remote RF Unit (RRU).

The RRU of distributed BTS integrates three RF units of traditional BTS: Transceiver and Receiver (TRX), Power Amplifier (PA) and RF Front End (RFE). With a protective enclosure that meets the IP55 standard, the RRU can be directly installed in an outdoor environment and in several modes: on a pole, on a wall, on a tower or in an integrated cabinet. Therefore, no equipment room space is required.

Being highly integrated, the BBU supports the same capacity as the baseband unit of traditional BTS but is much smaller. In an indoor environment, it can be installed on a wall or on an existing rack; thus, no extra indoor space is occupied. In an outdoor environment, it can be easily placed into the power supply cabinet or transmission cabinet of the BTS.

The distributed BTS is suitable for various installation scenarios as shown in Figure 2. If the equipment room has space constraints, the BBU can be installed in the equipment room, while the RRU is installed outdoors together with the antenna. In case the indoor installation space is not available, the BBU can be installed in the outdoor accessory cabinet, where the accessory equipment such as power supply, battery and transmission are placed, and the RRU can be directly installed outdoors, either with the antenna or in an integrated BBU & RRU cabinet. This integrated cabinet and the accessory cabinet can be installed on the ground or on top of the building; consequently, no indoor space is required. In the case of indoor coverage, the BBU can be installed in the basement or corridor, while the RRU can be mounted on the wall of the storey to be covered.

To support a smooth evolution in the future, ZTE adopts the Soft Defined Radio (SDR) technology in its distributed BTS, called distributed soft BTS. With this technology, the BTS can be applied in a 4G network by only upgrading the software or replacing some baseband boards. ZTE's distributed soft BTS has the following attractions:

Real SDR technology supports hardware of different standards, maximally protecting operators' investment.The BBU can support different standards such as CDMA, WCDMA, GSM, WiMAX, UMB and LTE by installing the related software. But the Channel Processing Module (CHM) board has to be replaced for other standards only when special chips are used on the board (at present, only the CDMA system uses special chips, i.e., Application-specific Integrated Circuit, or ASIC). As to the RRU, if the frequency band remains unchanged, it is only necessary to install the related software when supporting a different standard.

A variety of BBU/RRU products deliver full coverage solutions, meeting operators' various requirements for network construction. The RRU products with different power specifications, such as 60 W, 40 W and 5 W, are available, and they can meet the coverage needs in urban dense areas, suburbs and countryside. Moreover, the 200 mW pico RRU can be applied for indoor coverage.

BBU/RRU products feature high integration, large capacity, small size and light weight, making easy installation and maintenance while reducing operators' CAPEX.With the standard 2U height design, the BBU can support a capacity of 36 carrier-sectors for the CDMA2000 1X or EV-DO system. The RRU, which is 16.5 kg in weight and has a dimension of 380 × 330.2 × 152.4 mm (H × W × D), can support a maximum capacity of 6 CDMA2000 1X or EV-DO carriers and a maximum transmit power of 60 W at the cabinet top. Both the BBU and RRU provide the highest integration among the like products in the industry.